From what I can tell, it's because the Open University courses take on a far wider range of applicants than traditional universities (from people who failed all their GCSEs, to people who already have a degree), and so they need to make use of the full spectrum of marks from 40% to 85% to produce any sort of meaningful distinction amongst the wide range of students. So yes, it is /relatively/ easy to obtain a higher percentage pass at the OU.
However, it is also worth bearing in mind that dividing universities into the sets
1. OU
2. all other universities
is unhelpful. Courses vary massively from university to university, some are far, far harder than others. Noone who knows a bit about the university system in the UK seriously claims that all degrees in a certain subject are equal and comparable across all institutions.